The problem:
This relates specifically to the newer NT backs. Because of the redesigned dark slide/shutter interlock, if the dark slide is removed as if to take a picture and if some pressure is applied to the shutter release button but no picture is taken, then it is not possible to reinsert the dark slide.

To me, the biggest problem with this situation is that you will not be able to remove the film back from the camera if you wanted to change films. There is an interlock to prevent you from accidentally removing the film back without the dark slide in place. And if you were to somehow force it off, you would fog the film and lose one or more exposures anyway. Not good.

Why does it happen?:
The problem comes from this pin which protrudes from the rear of the camera body when the shutter button is depressed. Unfortunately, on the factory Kiev 88 and 88CM (I'm not sure about Kiev USA or Hartblei models) this pin does not retract when pressure on the shutter button is release. It only retracts after the shutter has been fired.

With the film magazine's dark slide in place, the hole in the lower left corner of the film magazine gets covered. When this hole is covered, the pin from the camera body is blocked and the shutter cannot fire (this is the safety interlock).

With the older film backs, this hole doesn't get blocked until the dark slide is completely inserted into the magazine. Even if the pin is sticking out from the back of the camera body, it is still possible to reinsert the dark slide sufficiently to allow removal of the film back.

However, the NT backs engage their interlock when the dark slide is first inserted. With the pin protruding from the camera, the interlock cannot engage, and the dark slide cannot be inserted. So even if you wanted to change film backs, you would not be able to do so.

A couple of possible solutions:
Since the NT backs have a double exposure function, it is possible to use it to save the frame of film. Simply set the magazine to double exposure mode and with the lens cap on (setting the camera to 1/1000 sec and the aperture to its minimum might also be good ideas) fire the shutter. The pin should then retract completely into the camera body.

Then with the magazine still in double exposure mode, wind the shutter. The shutter will recock but the film will not advance. The double exposure switch should automatically return to single exposure mode. And you should then be able to reinsert the dark slide.

One other way which will work with regular Kiev 88's but not 88CM's, is to remove the lens from the camera and look inside of the body, behind the shutter release button. You will notice a lever connected to the shutter release button, and if you pull that level towards the front of the camera with a straightened out paperclip or other such skinny thing, you can retract the pin from the back of the camera, allowing you to reinsert the dark slide. On 88CM's the floorplate in the camera body completely covers this area.

Going a bit further:
Caution! This is more of an essay (including some mistakes in the process) than a simple How-To. Please read the whole thing before trying it yourself.

To me, using the double exposure switch is a little troublesome and a bit time consuming if you're in a rush, and it does introduce the slight possibility of fogging the film a bit if done on a bright sunny day.

Wanting to lick the problem once and for all, I went a bit further and modified my two NT backs' interlocks. If you want to try this yourself, I recommend doing it on a clear work surface and being careful not to lose any of the small pieces.

First, I removed the ten small screws that hold the film magazine's cover in place (they're all the same, so you don't have to worry about keeping track of which one goes where... just don't lose any of them!). It might be a good idea to remove the screws on from the bottom last, and keep pressure on the cover until you're ready to remove it.

As a safety precaution, it would also be a good idea to place a small, light towel over the whole magazine when you remove the cover. There is a small spring (red arrow) that sometimes flies out when the cover is removed. I didn't know this the first time and thus spent a few hours crawling around the kitchen on my hands and knees looking for the missing spring. Also, be careful not to lose the two silver tabs that help to hold the magazine to the camera body (blue arrows).

With the cover removed, you will see the oddly shaped piece which is the safety interlock. At first I simply removed the whole piece and the spring and replaced the cover. The problem is that with no interlock, it is possible to fire the shutter with the dark slide still in the magazine. No pictures.

I thought I'd just be careful and be sure to remove the dark slide before getting down to the business of taking pictures. But, well, it's easy to forget. And the next day, as if to prove the point, I proceeded to shoot half a roll of film with the dark slide still in place. Duh.

Back to the drawing board. I realized that I did want the interlock, so I had to modify it but still keep it functioning.

My first approach was to simply snip off the tab at the end which gets engaged when the dark slide is first inserted into the magazine. This works to some extent. However, the spring which disengages the interlock is VERY weak, and perhaps without that tab in place to drag against the dark slide when its removed, the interlock was often left engaged. This meant that I couldn't fire the shutter even when the dark slide is removed. Not a good side effect of the operation.

So, I tried simply leaving out the screw closest to the spring when re-attaching the magazine's cover -- allowing the spring more freedom to move. However, this introduced a light leak from that spot. No good.

Next try. I removed the cover again and using the edge of a file, I scraped away some metal from the cover to make the groove for the spring deeper. This worked. But be careful not to remove too much metal. The cover is thin and you don't want to open up a hole in it.

The shutter interlock now works and there are no light leaks.

Even if the shutter button was slightly depressed and that pin is sticking out from the back of the camera, I can still insert the dark slide enough to protect the film, and with a little fiddling I can remove the magazine from the camera body. Mission accomplished!

Perhaps a better way:
With my second NT back, keeping in mind how weak the spring is, I didn't simply snip off the tab at the end of the interlock piece. I wanted some of it to remain so that it would still drag against the dark slide when being removed, and help to disengage the interlock.

So, instead of snipping it off completely, I cut only half of it off which allows it to move downward and out of the way earlier -- allowing the dark slide to go in. Then with a file I reshaped the top a bit and smoothed it out so that the dark slide would not hang up on it. But the tip still remains in contact with the dark slide and does seem to help in disengaging the interlock when the dark slide is removed.

In hind sight, this approach seems better than simply cutting the whole thing off.

Conclusion:
Now I have two NT backs which I use happily. I don't have to worry about the dark slide/shutter interlock problem anymore or having to fiddle with the multiple exposure switch. If you're inclined to try this yourself, I'd say "Go for it!" and please let me know how it works for you.

Good luck!